In a contemporary telephone system, a telephone user can address a radio telephone unit or a land-line telephone by entering a telephone number that corresponds to the phone system address of the individual to be contacted. Once a communication connection is complete, two way voice or data communication can take place.
However, with the growing use (system loading) of portable radio telephone (Cellular or Cordless Telephone Two (CT-2)) systems, there are primarily two conditions (problems) that can prevent the establishment of a radio frequency communication link with a base site (e.g., telepoint): all channels of the base site may be busy handling other calls, or the portable radio telephone attempting to access the base site may be out-of-range.
In typical CT-2 type communication systems, the number of available channels are limited, typically forty, and the receivers are designed with low adjacent channel selectivity (e.g., 20 dB), making it impossible to use adjacent channels at a single base site for communication. As a result, a base site cannot accommodate more than twenty contemporaneously active channels despite being equipped with forty channels. That is, conventional CT-2 communication systems can only support contemporaneous communication on one-half of the communication channels available for use.
Operationally, a CT-2 communication system requires each handset (communicator) originating a call to select a channel based on the lowest measured signal strength on all forty channels. Unfortunately, the somewhat random nature of the channel selection process may further reduce the number of channels available for use by a base site. That is, as the requirement for additional channels increases, the random channel selection process will often select two "in-use" channels having two "unused" adjacent channels therebetween. This problem is illustrated in FIGS. 1A and 1B.
FIG. 1A shows a diagram of forty available channels, but because of the limits set by the low adjacent channel selectivity, the maximum number (best case) of channels available for use is twenty (i.e., using alternate channels). FIG. 1B, further illustrates the problem associated with the random selection of channels based on the lowest measured interference. According to the diagram, the worst case condition is shown, where:
R=Real signal (i.e., channels that are active); PA1 U=Unusable adjacent channel. PA1 (a) generating a plurality of interference signals in a predetermined pattern on a subset of communication channels of a plurality of communication channels; PA1 (b) assigning an initiating one of a plurality of communicators to a communication channel being substantially free of interference for communicating via the assigned communication channel; PA1 (c) removing selected interference signals in response to step (b); and PA1 (d) terminating communicating on the assigned channel; and (e) replacing the interference signals in response step (d) whereby the steps of removing and replacing further steer initiating communicators to select communication channels according to the pattern of interference signals being generated on the subset of the plurality of communication channels.
When the "inuse" (active channels) are selected according to the positions R of FIG. 1B, the total available channels are reduced from twenty channels to fourteen channels. This is because the random selection process can only ensure that the channel being selected is not adjacent to an in-use channel. Thus, once a channel is determined to have the lowest signal strength, it is selected by a radio telephone so long as if it is not adjacent to an "inuse" channel. Regrettably, this process often results in two adjacent channels between two "inuse" channel, resulting the reduced availability of channels illustrated by FIG. 1B.
Thus, what is needed is a method for selecting communication channels that maximizes the number of contemporaneous available communication channels.